The present invention relates to cross-linked epoxy resins and to their production and use.
Non-linear optics is concerned with the interaction between the electromagnetic field of a light wave and a medium in which the light wave is propagated, and with the related creation of new fields having changed properties. If the electromagnetic field interacts with the medium consisting of one molecule or of many molecules, then this field polarizes the molecules.
The polarization which is induced by a local electric field in a molecule can be represented--according to equation (1)--as an exponential series of the electrical field strength: EQU P=.alpha..multidot.E+.beta..multidot.E.sup.2 +.gamma..multidot.E.sup.3 +. . . (1);
P being the induced polarization and E the induced local electrical field while .alpha., .beta.and .gamma. represent the polarizability of the first, second and third order.
A similar relationship applies on the macroscopic plane--according to equation (2)--for the polarization induced by an external electrical field in a medium consisting of several molecules: EQU P=.epsilon..sub.o (.chi..sup.(1) .multidot.E+.chi..sup.(2) .multidot.E.sup.2 +.chi..sup.(3) .multidot.E.sup.3 +. . . )(2);
P is in this case again the induced polarization and E the induced local electrical field, .epsilon..sub.o is the dielectric constant, and .chi..sup.(1), .chi..sup.(2), and .chi..sup.(3) represent the dielectric susceptibility of the first, second and third order.
The dielectric susceptibilities of equation (1) have a similar meaning as the molecular coefficients of equation (1): They are material constants which depend on the molecular structure and the frequency and in general also on the temperature. Materials having a dielectric susceptibility of the second order are suitable for frequency doubling: This is is the conversion of light of a frequency .omega. into a light of the frequency 2.omega.. Another non-linear optical effect of the second order is the linear electrooptical effect (Pockels-effect); it results from the change in the index of refraction of the optical medium when the electrical field is applied. The optical rectification as well as the sum and difference frequency mixing are further examples of non-linear optical effects of the second order. Fields of use for materials of the above-mentioned type are, for instance, electrooptical switches and areas of information processing and integrated optics, such as optical chip-to-chip connections, wave-guiding in electrooptical layers, Mach-Zehnder-Interferometers and the optical signal processing in sensor technology.
Materials having a dielectric susceptibility of the third order are suitable for frequency tripling of the incident light wave. Further effects of the third order are the optical bistability and phase conjugation. Concrete examples for applications are holographic data processing and purely optical switches for the designing of purely optical computers.
In order to obtain a sufficient non-linear optical effect of the second order, the dielectric susceptibility of the second order .chi..sup.(2) must be greater than 10.sup.-9 electrostatic units (esu); this means that the hyperpolarizability .beta. must be greater than 10.sup.-30 esu. Another fundamental prerequisite for obtaining a non-linear optical effect of the second order is the non-centrosymmetric orientation of the molecules in the non-linear optical medium; otherwise, we namely have .chi..sup.(2) =0. This can be achieved, unless predetermined by the crystal structure, as in the case of crystalline materials, by an orientation of the molecular dipoles. Thus, the highest values of .chi..sup.(2) for a non-linear optical medium have been obtained by orientation of the molecular dipoles in electrical fields.
Inorganic materials such as lithium niobate (LiNbO.sub.3) and potassium dihydrogen phosphate(KH.sub.2 PO.sub.4), have non-linear optical properties. Semiconductor materials, such as gallium arsenide (GaAs), gallium phosphide (GAP) and indium antimonide (InSb), also have non-linear optical properties. Aside from the advantage of a high electrooptical coefficient of the second order, inorganic materials of the above-mentioned type, however, have some decisive disadvantages. Thus, the processing of these materials is technically very expensive since individual process steps are time-consuming and must be carried out with maximum precision (see in this connection: C. Flytzanis and J. L. Oudar "Nonlinear Optics: Materials and Devices", Springer Publishing Co. (1986), pages 2-30). Such materials are furthermore unsuitable for electrooptical components which operate at high modulation frequencies. Due to the intrinsically present high dielectric constants, there occur namely at high frequencies (above several GHz) such high dielectric losses that working at such frequencies is impossible (see in this connection: "J. Opt. Soc. Am. B", Vol. 6 (1989), pages 685-692).
It is known that organic and polymeric materials with extended .pi.-electron systems, which are substituted with electron donors and acceptors, can be used in non-linear optical media (see in this connection: R. A. Hann and D. Bloor "Organic Materials for Non-linear Optics", The Royal Society of Chemistry (1989), pages 382-389 and 404-411). Monocrystals on an organic base have a high electrooptical coefficient of the second order and good photochemical stability as compared with LiNbO.sub.3. The required high orientation of the non-linear optical molecules is also already present. However, some important criteria speak against industrial utilization of this category of materials. Thus, there is required for producing the monocrystals, both from a solution and from the melt, a period of 14-30 days (see in this connection: D. S. Chemla and J. Zyss "Nonlinear Optical Properties of Organic Molecules and Crystals", Academic Press, Inc. (1987), Vol. 1, pages 297-356). The production process thus does not meet the requirements of industrial production. Additionally, the melting point of the crystals is on the average around 100.degree. C. so that a working temperature range up to 90.degree. C. can probably not be realized. Furthermore, organic crystals cannot be structured and their lateral dimensions are at present still too small to permit designing as an electrooptical component.
Polymeric materials have recently become increasingly important as materials for applications of non-linear optics in the fields of information transmission and integrated optics. These polymeric materials can be produced in the manner that an external electrical field is applied to a specimen which has been heated to above the glass transition temperature; this leads to an orientation of the non-linear optical molecules. After cooling the polymer specimen to below the glass transition temperature, while the electrical field is applied, an anisotropic and thus a non-centrosymmetrical polymer is obtained which has dielectric susceptibilities of the second order.
Non-linear optical compounds which are dissolved in polymers or diffused into polymers can be worked into thin layers as is required for integrated optics (see in this connection: "Macromolecules", Vol. 15 (1982), pages 1385 to 1389; "Appl. Phys. Lett.", Vol. 49 (1986), pages 248 to 250; Electron. Lett.", Vol. 23 (1987), pages 700 to 701). However, the low solubility of the low-molecular compounds, their insufficient distribution in the polymers, the migration of the active molecules out of the polymer matrix and the loss of the non-centrosymmetric orientation of the active molecule species even at room temperature has a disadvantageous effect.
There are also known as non-linear optical compounds polymers having covalently bound non-linear optical molecule components which have at the same time liquid crystal character (see in this connection: EP-OS O 231 770 and EP-OS O 262 680). These materials do not have the above-mentioned disadvantages. However, they are not suitable at the present stage of development for applications in electrical or integrated optics, since in this case optical losses of more than 20 dB/cm result which are caused by the inherent domain scattering. Furthermore, investigations on amorphous non-linear optical polymers have already been reported (see: "Macromolecules, Vol. 21 (1988), pages 2899 to 2901).
Both in the case of liquid crystalline and amorphous polymers with covalently bound non-linear optical molecule units, a much higher concentration of such molecule units can be realized. In this case, a spacer uncouples the molecular mobility of the non-linear optical units from the polymer chain. At the same time, however, the glass transition temperature is drastically reduced. In the case of operating temperatures within the range of the glass transition temperature of the polymers, there must thus be expected the loss of the molecular orientation of the non-linear optical molecule units and the loss of the non-linear optical activity.